填空题 . 
    The Pacific Ocean is covered by a vast archipelago of islands that are spread out in long chains covering thousands of kilometers with some close to large landmasses and others much farther away. When European explorers chanced upon these islands, almost all of them were inhabited, and most had significant amounts of vegetation as well as large populations of various species of animals. Because nearly all of these islands were created by volcanic activity, such life—both plant and animal—would have been absent from them when they rose above the ocean's surface. The plants and animals living on them migrated from elsewhere by both air and sea. In many cases, they accomplished this on their own, but some of them got helping hands from humans.
    Plant life on the desolate volcanic Pacific islands most likely initially arrived due to the wind. Small seeds such as thistle seeds and the spores of ferns are lightweight enough to have been carried aloft great distances whereas heavy seeds would have had more difficulty being transported that way, especially to remote, isolated islands such as Easter Island and the Galapagos Islands. The latter group has many lichens, ferns, and mosses, which all grow from light spores, yet it has few vascular plants, which have heavier seeds. As for vascular plant life, it could have arrived on other islands through two ways: by water or birds. The coconut tree is widespread throughout the Pacific islands in spite of its enormous seeds being too heavy to be carried by the wind, yet it is light enough to float, so it has moved all across the ocean in that way. Birds may have also consumed various heavy seeds, flown to islands, and then defecated the seeds onto these new lands, whereupon they then began to grow. There may not have been suitable soil for seeds to grow in on these volcanic islands at first, but, over time, as new plant life arrived, the soil was sufficiently broken up and filled with nutrients, thereby allowing many plant species to take root.
    Similarly, animals arrived by air and sea. The birds that first arrived on the Pacific islands indisputably flew from nearby larger landmasses. Over many generations, they could have island-hopped from one place to another. Small insects—and possibly tiny invertebrates—may have been light enough to be windblown to various islands. Certainly, sea creatures such as turtles and penguins arrived on the islands by swimming to them. Small mammals and reptiles may have arrived on them by water, most likely after floating on rafts of dense vegetation that were blown out to sea during strong storms. Some species of mice have been known to do this, and lizards have also been found on rafts far from their native homelands.
    Today, many Pacific islands are home to a wide variety of plants and animals that reside throughout the rest of the world. The reason is that they were brought to the islands as a result of human migration. Over thousands of years, people spread from mainland Asia into the Pacific islands as they reached as far south as New Zealand, as far east as Easter Island, and as far north as Hawaii. During these great migrations, people took their plants and animals with them. One example is the chicken, which is found virtually everywhere in the Pacific today because it was transported in large canoes from island to island. The pig was also widespread throughout the Pacific and became an important source of protein for Pacific islanders. Some animals, such as mice, were most likely stowaways on islanders' canoes. Furthermore, people took their staple plants, such as taro and yams, which they planted on the islands as they slowly made their way across the Pacific Ocean.
    The Pacific natives were not the only ones who contributed to animal and plant migration as Europeans also played a role of their own. Unfortunately, European explorers brought many animals that caused disruptions on the islands. ❶Dogs, rats, snakes, and cats, for instance, hunted many species of small mammals and birds to extinction. ❷Plant eaters such as goats and sheep had negative effects on small islands' vegetation as well. ❸Nevertheless, these new animals provided some benefits. ❹Goats and sheep, for instance, became important to the settlements of Europeans on New Zealand.28.  in paragraph 1, the author implies that many of the Pacific islands ______